1. Field of the Invention
The present invention relates to a process for preventing and removing obstructions in the outlet opening of a metering slot of a coating apparatus.
2. Description of the Related Art
Coating apparatuses serve the function of applying a liquid or viscid coating medium onto a moving material web such as paper or carton. Coating apparatuses are well-known, and the following references serve to provide additional documentation: U.S. Pat. Nos. 5,436,030 and 5,858,096, EP patent document nos. 0,466,420 and 0,701,022, and German patent document nos. 9,417,321 and 9,417,324. These publications describe free jet coating apparatuses, also known as fountain applicators or JetFlow F, having a metering slot between two protruding surfaces, hereinafter referred to as lips. The coating medium is supplied to the metering slot by a distribution pipe residing within a girder which extends across the width of the coating apparatus. The coating medium travels from the distribution pipe through passages to the metering slot from which the coating medium exits in the form of a free jet.
In the case of direct application, the material web runs by the metering slot and the coating medium is directly applied onto the web. In these cases, the material web is typically guided by the outer surface of a support roll. In the case of indirect application, the coating medium is first applied onto the surface of an applicator roll and is subsequently transferred onto the material web in the nip area through which the web traverses.
The lip which resides on the side of the metering slot facing the oncoming web is referred to as "inboard" lip. Accordingly, the lip which resides on the outboard side of the metering slot is referred to as the "outboard" lip.
Inside the exit opening of the metering slot, or the jet exit opening, there is a tendency for dirt particles, lumps or other undesirable solid matter particles to collect, causing the coating medium that is exiting the metering slot to "break up", resulting in an insufficient coating on the material web or, worse yet, allowing the contamination onto the web itself. This condition results in defects in the coating that has been applied to the material web. Trained machine operators are relied upon to recognize and detect these defects. The cause of these defects (i.e., undesirable particles within or expelled from the metering slot) must be subsequently removed from the metering slot by means of a scraper, such as a piece of plastic strip. This process, however, is very time-consuming and can only be performed when the coating apparatus is not operating. Furthermore, this task requires skill and experience on the part of the machine operator.
These defects, however, can also be detected by means of a specially-designed sensing device which then generates the appropriate alert signals. In order to respond to the signals with appropriate counter measures, upon recognition of a defect on the coating at least one operating parameter of the coating apparatus must be adjusted or modified in an attempt to dislodge the obstructions from the metering slot while the material web continues to run. Specifically, this process and the corresponding devices require the manipulation, either temporarily or repeatedly, of operating parameters, such as the coating medium pressure or the width of the metering slot, which affect the characteristic features of the jet of coating medium exiting the metering slot or the shape of the jet pattern. A disadvantage of this approach is that the coating apparatus must always be re-adjusted (at least approximately) to the initial parameter settings, which requires additional resources.